In the construction of the heat exchanger, it is common to work the free end of the tubes, sometimes referred to as the tube studs. Such working may be upsetting the nominal tube inner diameter (ID) to a larger ID to provide a braze cup, for axially receiving a further conduit. Since other elements of the heat exchanger core, such as fin plates and end plates, must be positioned over the heat exchanger tubes in close physical contact thereto prior to the upsetting operation, the formation of the braze cup must take place in situ.
The prior art tooling utilized to accomplish the braze cup expansion operation frequently is not properly guided relative to the tube axis to assure that the resulting braze cup is in centered alignment with the tube axis. Some prior art guiding mechanisms currently used to maintain any alignment such as taught by U.S. Pat. Nos. 4,584,765 and 4,584,751 (both issued Apr. 29, 1981) depend on theoretical tube location and are subjected to metal on metal sliding, which through repeated use allows successive clearances to accumulate, further comprising the alignment. Such manufacturing operations are not guided by the actual position of the heat exchanger tubes and require separate locating means such as collet plates. This results in the braze cups being formed on an axis which is different from the tube axis.
U.S. Pat. No. 4,327,567 issued May 4, 1982, teaches working a boiler tube end to provide a flange and wherein the working tool includes a mandrel having radially movable rollers. While the rollers engage the tube bore in the plane of a tube sheet, this does not necessarily assure alignment of the mandrel. Furthermore, Martin makes no statements of the problems of centering the worked tube end relative to a tube axis, particularly utilizing a tube end working tool centered around an expansion rod which has previously been forced through the tube as part of another operation. The Martin device, directed to elimination of false indications of torque, requires rotation of the mandrel due to the roller construction, which would require needless expensive tooling and rotation equipment relative to normal operations in making heat exchanger cores.
Braze cup misalignment causes two specific problems in constructing the heat exchanger. First, when multiple tubes are to be joined by a single preshaped form, insertion into the multiple braze cups may be difficult or impossible if the misalignment is not in the same direction and of the same magnitude. A simple example is a pair of tube braze cups to be joined by a return bend. The axes of the two ends of the preformed return bend are spaced at the theoretical distance between the axes of two heat exchanger tubes. The ID of the braze cups of the pair of tubes are designed to snuggly receive the two ends of the return bend. If the braze cups are misaligned relative to the tube axis by even a few thousandths inch, especially if the misalignment occurs in opposite directions, insertion of the return tube may be impossible. This may require an insertion aid (i.e. a hammer) and/or other manual force to be applied to fully insert and seat the mating parts. Such extra manual operations, which are not only labor intensive and thus expensive in a highly competitive field, can cause uneven distortion of the respective parts. Furthermore, the complexity of such operation is magnified when many tube braze cups are to be joined by a single complex shaped fluid conduit means such as a return header having multiple independent conduits.
Secondly, in normal manufacturing technique, the mating parts are brazed to the tube braze cups. Misalignment between the braze cup axis and the axis of the mating part can result in improper radial clearance to permit proper capillary flow of the brazing alloy, which can result in a defective, or leaking, joint. Ideally, a relatively uniform radial gap between the return bend or other conduit outer diameter (OD) and the braze cup ID is required for uniform brazing. This cannot occur when the brace cups are misaligned relative to the tube axis, even if the return bend can be inserted without deformation.
As the heat exchanger industry is changing to more modern manufacturing equipment, including automatic insertion of end pieces such as the return bends, such misalignment problems become more critical. At least with the previous manual insertion, the operator is aware when difficulties are encountered. Automatic operation machinery is not sensitive to such problems, resulting in an increased number of defective joints. Furthermore, under automatic maufacturing concepts, the requirement for manual intervention is even less tolerable.